This patch adds grid supports to slivers and introduces a ScrollGrid
convenience class for making the common types of scrollable grids.

This patch also deploys ScrollGrid in an example in the Flutter Gallery.

The finder will only match the widget's ValueKey if both have identical
runtime types

Our previous behavior of hard-coding the unselected label color to 70% of the
selected color was too restrictive.

Fixes #7738

Now that ScrollView supports itemExtent, we can use it in a few more places.

Add SliverList

A SliverList is a linear layout of box children in a viewport that all
have a common, fixed extent along the scroll axis. The layout is similar
to a SliverBlock but more efficient.

Fixes https://github.com/flutter/flutter/issues/4620

Fixes for: https://github.com/flutter/flutter/issues/7570 https://github.com/flutter/flutter/issues/7231 https://github.com/flutter/flutter/issues/2841 and others

ValueKey and ObjectKey shouldn't be == with subclasses.
Fixes https://github.com/flutter/flutter/issues/3107

Clean up toString for the keys a bit.

Add a test for keys.

The needsLayout getter is prone to misuse. See discussion on #3083.

Reverts a change from https://github.com/flutter/flutter/pull/3158.
Fixes https://github.com/flutter/flutter/issues/3161

fixes https://github.com/flutter/flutter/issues/7654

Combines many sliver features at once.

This structure makes it easier to customize the viewport used in a Scrollable.
Now Scrollable2 doesn't commit to using slivers.

Responding to https://github.com/flutter/flutter/pull/7640#pullrequestreview-18563416

Currently, this only calls out to the PowerShell Script, which is kind of slow. In the future, we will transfer more logic from PowerShell to cmd for faster starup time.

This remove a very brittle aspect of flutter drive, whereby it would
assume a known port instead of explicitly finding out what it was.

Fixes #7692 and hopefully fixes the devicelab tests.

Move the drive/run common stuff to RunCommandBase, rather than the
superclass which applies to everything.

Implement --no-resident (fixes
https://github.com/flutter/flutter/issues/5448). This is particularly
useful when running flutter from scripts.

Rename RunAndStayResident to ColdRunner.

Uses SliverPadding to implementing paddding in ScrollView. Also, deploy
ScrollView in more places now that it implements padding.

Finally, remove loader_app.dart because it is not referenced.

We now report an accurate estimate when the lastIndex is the final index
in the list.

Also, we now project forward from the lastIndex using the extent of the
reified children instead of trying to project forward from index 0.

This new algorithm leads to estimation error as you reach the end of a
list, avoiding some ballistic overscrolls.

...for the port arguments

Rumours have it that this is faster now.

Fixes https://github.com/flutter/flutter/issues/5154

Turns out MacOS has a really low open files limit and so if you try to
open EVERY FILE AT THE SAME TIME it falls over.

This just opens the files one at a time, the way we used to back in
the old days.

Almost all real-world apps will want a custom app delegate, and 'flutter
create' code-gens one by default. This brings the samples in line with
our templates and the most common use case.

When we start a ballisitic scroll activity to correct an out-of-range scroll
offset, make sure we start with a velocity that is headed in the right
direction.